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420 !!!!! Concrete Technology

Testing ofHardenedConcrete

10C H A P T E R

TTTTT esting of hardened concrete plays animportant role in controlling and confirming the

quality of cement concrete works. Systematic testingof raw materials, fresh concrete and hardenedconcrete are inseparable part of any quality controlprogramme for concrete, which helps to achievehigher efficiency of the material used and greaterassurance of the performance of the concrete withregard to both strength and durability. The testmethods should be simple, direct and convenient toapply.

One of the purposes of testing hardenedconcrete is to confirm that the concrete used at sitehas developed the required strength. As thehardening of the concrete takes time, one will notcome to know, the actual strength of concrete forsome time. This is an inherent disadvantage inconventional test. But, if strength of concrete is to beknown at an early period, accelerated strength testcan be carried out to predict 28 days strength. Butmostly when correct materials are used and carefulsteps are taken at every stage of the work, concretes

420

""""" Compression Test

""""" The Flexural Strength of Concrete

""""" Factors influencing the strength results

""""" Test Cores

""""" Relationship between Pulse Velocity and Static Young’s Modulus of Elasticity

""""" Combined Methods""""" Radioactive Methods

""""" Nuclear Methods

""""" Magnetic Methods

""""" Electrical Methods

""""" Tests on Composition of Hardened Concrete

""""" Determination of Cement Content

""""" Determination of the Original Water/ Cement Ratio

""""" Physical Method

""""" Accelerated Curing Test

Concrete testing and Diagnosing Kit

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Testing of Hardened Concrete !!!!! 421

normally give the required strength. The tests also have a deterring effect on those responsiblefor construction work. The results of the test on hardened concrete, even if they are knownlate, helps to reveal the quality of concrete and enable adjustments to be made in theproduction of further concretes. Tests are made by casting cubes or cylinder from therepresentative concrete or cores cut from the actual concrete. It is to be remembered thatstandard compression test specimens give a measure of the potential strength of the concrete,and not of the strength of the concrete in structure. Knowledge of the strength of concretein structure can not be directly obtained from tests on separately made specimens.

Compression TestCompression test is the most common test conducted on hardened concrete, partly

because it is an easy test to perform, and partly because most of the desirable characteristicproperties of concrete are qualitatively related to its compressive strength.

The compression test is carried out on specimens cubical or cylindrical in shape. Prismis also sometimes used, but it is not common in our country. Sometimes, the compressionstrength of concrete is determined using parts of a beam tested in flexure. The end parts ofbeam are left intact after failure in flexure and, because the beam is usually of square crosssection, this part of the beam could be used to find out the compressive strength.

The cube specimen is of the size 15 x 15 x 15 cm. If the largest nominal size of theaggregate does not exceed 20 mm, 10 cm size cubes may also be used as an alternative.Cylindrical test specimens have a length equal to twice the diameter. They are 15 cm indiameter and 30 cm long. Smaller test specimens may be used but a ratio of the diameter ofthe specimen to maximum size of aggregate, not less than 3 to 1 is maintained.

Cube beam and cylinder moulding


MouldsMetal moulds, preferably steel or cast iron, thick

enough to prevent distortion are required. They aremade in such a manner as to facilitate the removal ofthe moulded specimen without damage and are somachined that, when it is assembled ready for use,the dimensions and internal faces are required to beaccurate within the following limits.

The height of the mould and the distancebetween the opposite faces are of the specified size± 0.2 mm. The angle between adjacent internalfaces and between internal faces and top andbottom planes of the mould is required to be 90° ±0.5°. The interior faces of the mould, are planesurfaces with a permissible variation of 0.03 mm.Each mould is provided with a metal base platehaving a plane surface. The base plate is of such dimensions as to support the mould duringthe filling without leakage and it is preferably attached to the mould by springs or screws. Theparts of the mould, when assembled, are positively and rigidly held together, and suitablemethods of ensuring this, both during the filling and on subsequent handling of the filledmould, are required to be provided.

In assembling the mould for use, the joints between the sections of the mould are thinlycoated with mould oil and a similar coating of mould oil is applied between the contactsurface of the bottom of the mould and the base plate in order to ensure that no waterescapes during the filling. The interior surfaces of the assembled mould is also required to bethinly coated with mould oil to prevent adhesion of concrete.

The cylindrical mould is required to be of metal which shall be not less than 3 mm thick.Each mould is capable of being opened longitudinally to facilitate removal of the specimenand is provided with means of keeping it closed while in use. Care should be taken so thatthe ends are not departed from a plane surface, perpendicular to the axis of the mould, bymore than 0.05 mm. When assembled ready for use the mean internal diameter of the mouldshould be 15.0 cm ± 0.2 mm. and in no direction the internal diameter be less than 14.95cm. or more than 15.05 cm. The height maintained is 30.0 cm ± 0.1 mm. Each mould isprovided with a metal base plate, and with a capping plate of glass or orther suitable material.The base plate and the capping plate are required to be at least 6.5 mm thick and such thatthey do not depart from a plane surface by more than 0.02 mm. The base plate supports themould during filling without leakage and is rigidly attached to the mould. The mould andbase plate are coated with a thin film of mould oil before use, in order to prevent adhesionof concrete.

A steel bar 16 mm in diameter, 0.6 m long and bullet pointed at the lower end servesas a tamping bar.

CompactingThe test cube specimens are made as soon as practicable after mixing and in such a way

as to produce full compaction of the concrete with neither segregation nor excessive laitance.The concrete is filled into the mould in layers approximately 5 cm deep. In placing each

Vibrating table for cubes


scoopful of concrete, the scoop is required tobe moved around the top edge of themould as the concrete slides from it, in orderto ensure a symmetrical distribution of theconcrete within the mould. Each layer iscompacted either by hand or by vibration.After the top layer has been compacted thesurface of the concrete is brought to thefinished level with the top of the mould, usinga trowel. The top is covered with a glass or metal plate to prevent evaporation.

Compacting by HandWhen compacting by hand, the standard tamping bar is used and the strokes of the bar

are distributed in a uniform manner over the cross-section of the mould. The number of strokesper layer required to produce the specified conditions vary according to the type of concrete.For cubical specimens, in no case should the concrete be subject to less than 35 strokes perlayer for 15 cm or 25 strokes per layer for 10 cm cubes. For cylindrical specimens, the numberof strokes are not less than thirty per layer. The strokes penetrate into the underlying layer andthe bottom layer is rodded throughout its depth. Where voids are left by the tamping bar, thesides of the mould are tapped to close the voids.

Compacting by VibrationWhen compacting by vibration, each layer is vibrated by means of an electric or

pneumatic hammer or vibrator or by means of a suitable vibrating table until the specifiedcondition is attained. The mode and quantum of vibration of the laboratory specimen shallbe as nearly the same as those adopted in actual concreting operations. Care must be takenwhile compacting high slump concrete which are generally placed by pumping. If care its nottaken severe segregation takes place in the mould, which results in low strength when cubesare crushed. The cube crushing strength does not represent the strength of the concrete.

Buoyancy Balance


Capping SpecimensCapping is applicable to cylindrical specimen. The ends of all cylindrical test specimens

that are not plane within 0.05 mm are capped. The capped surfaces are not departed froma plane by more than 0.05 mm and shall be nearly at right angles to the axis of the specimens.The planeness of the cap is required to be checked by means of a straight edge and feelergauge, making a minimum of three measurements on different diameters. Caps are made asthin as practicable and care should be taken so that flaw or fracture does not take place, whenthe specimen is tested. Capping can be done on completion of casting or a few hours priorto testing of specimen. Capping is required to be carried out according to one of the followingmethods:

(a) Neat Cement:Neat Cement:Neat Cement:Neat Cement:Neat Cement: The test cylinders may be capped with a thin layer of stiff, neat Portlandcement paste after the concrete has set in the moulds: Capping is done after about4 hours of casting so that concrete in the cylinder undergoes plastic shrinkage andsubsides fully. The cap is formed by means of a glass plate not less than 6.5 mm inthickness or a machined metal plate not less than 13 mm in thickness and having aminimum surface dimension at least 25 mm larger than the diameter of the mould.It is worked on the cement paste until its lower surface rests on the top of the mould.The cement for capping is mixed to a stiff paste for about 2 to 4 hours before it is tobe used in order to avoid the tendency of the cap to shrink. Adhesion of paste to thecapping plate is avoided by coating the plate with a thin coat of oil or grease.

(b) Cement Mortar:Cement Mortar:Cement Mortar:Cement Mortar:Cement Mortar: On completion of casting cylinder, a mortar is gauged using cementsimilar to that used in the concrete and sand which passes IS Sieve 300 but is retainedon IS Sieve 150. The mortar should have a water/cement ratio not higher than thatof the concrete of which the specimen is made, and should be of a stiff consistency.If an excessively wet mix of concrete is being tested, any free water which hascollected on the surface of the specimen shouldbe removed with a sponge, blotting paper orother suitable absorbant material before the capis formed. The mortar is then applied firmly andcompacted with a trowel to a slightly convexsurface above the edge of the mould, afterwhich the capping plate is pressed down on thecap with a rotary motion until it makes completecontact with the rim of the mould. The plateshould be left in position until the specimen isremoved from the mould.

(c ) Sulphur:Sulphur:Sulphur:Sulphur:Sulphur: Just prior to testing, the cylindricalspecimens are capped with a sulphur mixtureconsisting of 1 part of sulphur to 2 or 3 parts ofinert filler, such as fire-clay. The specimens aresecurely held in a special jig so that the capsformed have a true plane surface. Care has tobe taken to ensure that the sulphur compoundis not over-heated as it will not then develop therequired compressive strength. Sulphur caps areallowed to harden for at least 2 hours beforeapplying the load.

Compression testing machine


(d ) Har Har Har Har Hard Plaster:d Plaster:d Plaster:d Plaster:d Plaster: Just prior to testing, specimens are capped with hard plaster having acompressive strength of at least 42 MPa cm in an hour. Such plasters are generallyavailable as proprietary material. The caps can be formed by means of a glass platenot less than 13 mm in thickness, having a minimum surface dimension at lest 25 mmlarger than the diameter of the mould. The glass plate is lightly coated with oil toavoid sticking. Ordinary plaster of paris will not serve the purpose of capping materialdue to its low compressive strength.

CuringThe test specimens are stored in place free from vibration, in moist air of at least 90%

relative humidity and at a temperature of 27° ± 2°C for 24 hours ± 1/2 hour from the time ofaddition of water to the dry ingredients. After this period, the specimens are marked andremoved from the moulds and unless required for test within 24 hours, immediatelysubmerged in clean fresh water or saturated lime solution and kept there until taken out justprior to test. The water or solution in which the specimens are submerged, are renewed everyseven days and are maintained at a temperature of 27° ± 2°C. The specimens are not to beallowed to become dry at any time until they have been tested.

Making and Curing Compression Test Specimen in the FieldThe test specimens are stored on the site at a place free from vibration, under damp

matting, sacks or other similar material for 24 hours ± 1/2 hour from the time of addition ofwater to the other ingredients. The temperature of the place of storage should be within therange of 22° to 32°C. After the period of 24 hours, they should be marked for lateridentification removed from the moulds and unless required for testing within 24 hours, storedin clean water at a temperature of 24° to 30°C until they are transported to the testinglaboratory. They should be sent to the testing laboratory well packed in damp sand, dampsacks, or other suitable material so as to arrive there in a damp condition not less than 24hours before the time of test. On arrival at the testing laboratory, the specimens are stored inwater at a temperature of 27° ± 2°C until the time of test. Records of the daily maximum andminimum temperature should be kept both during the period the specimens remain on thesite and in the laboratory particularly in cold weather regions.

Failure of Compression SpecimenCompression test develops a rather more complex system of stresses. Due to compression

load, the cube or cylinder undergoes lateral expansion owing to the Poisson’s ratio effect. Thesteel platens do not undergo lateral expansion to the some extent that of concrete, with theresult that steel restrains the expansion tendency of concrete in the lateral direction. Thisinduces a tangential force between the end surfaces of the concrete specimen and theadjacent steel platens of the testing machine. It has been found that the lateral strain in thesteel platens is only 0.4 of the lateral strain in the concrete. Due to ths the platen restrainsthe lateral expansion of the concrete in the parts of the specimen near its end. The degreeof restraint exercised depends on the friction actually developed. When the friction iseliminated by applying grease, graphite or paraffin wax to the bearing surfaces the specimenexhibits a larger lateral expansion and eventually splits along its full length.

With friction acting i.e., under normal conditions of test, the elements within thespecimen is subjected to a shearing stress as well as compression. The magnitude of the shearstress decreases and the lateral expansion increases in distance from the platen. As a result ofthe restraint, in a specimen tested to destruction there is a relatively undamaged cone of


height equal to 3

2 d (where d is the lateral dimension of the specimen).10.1 But if the

specimen is longer than about 1.7 d, a part it of will be free from the restraining effect of theplaten. Specimens whose length is less than 1.5 d, show a considerably higher strength thanthose with a greater length. (See Fig. 10.1).


Effect of the Height/Diameter Ratio on StrengthNormally, height of the cylinder “h” is made twice the diameter “d”, but sometimes,

particularly, when the core is cut from the road pavements or airfield pavements orfoundations concrete, it is not possible to keep the heigh/diameter ratio of 2:1. The diameterof the core depends upon the cutting tool, and the height of the core will depend upon thethickness of the concrete member. If the cut length of the core is too long. It can be trimmedto h/d ratio of 2 before testing. But with too short a core, it is necessary to estimate thestrength of the same concrete, as if it had been determined on a specimen with h/d ratioequal to 2.

Fig. 10.2 shows the correction factor for height/diameter ratio of a core (IS 516.1959).

Murdock and Kesler10.2 found that correlation factor is not a constant one but dependson the strength level of concrete. High strength concrete is less affected than the low strengthconcrete. Figure 10.3 shows the influence of h/d ratio on the strength of cylinder for differentstrength levels.

Figure 10.1 shows the general pattern of influence of h/d ratio on the strength ofcylinder.

It is interesting to note that the restraining effect of the platens of the testing machineextends over the entire height of the cube but leaves unaffected a part of test cylinder becauseof greater height. It is, therefore, the strength of the cube made from identical concrete willbe different from the strength of the cylinder. Normally strength of the cylinder is taken as 0.8times the strength of the cube, but experiments have shown that there is no uniquerelationship between the strength of cube and strength of cylinder. It was seen that thestrength relation varies with the level of the strength of concrete. For higher strength, thedifference between the strength of cube and cylinder is becoming narrow. For 100 MPaconcrete the ratio may become nearly 1.00. Table 10.1 shows the strength pattern of cubesand cylinders.


Table 10.1. Strength of Cubes and Cylinders

Ratio of DifferenceCompressive Strength strengths of strength

cylinder/ cube (cube-cylinders)

Cube CylinderMPa MPa MPa

9.0 7.0 0.77 216.0 12.0 0.77 420.0 16.0 0.76 425.0 20.0 0.81 528.0 25.0 0.87 330.0 27.0 0.91 330.0 28.0 3.91 236.0 32.0 3.89 437.0 35.0 0.94 243.0 37.0 0.87 645.0 42.0 0.92 349.0 45.0 0.91 454.0 51.0 0.96 3

Comparison between Cube and Cylinder StrengthIt is difficult to say whether cube test gives more realistic strength properties of concrete

or cylinder gives a better picture about the strength of concrete. However, it can be said thatthe cylinder is less affected by the end restrains caused by platens and hence it seems to givemore uniform results than cube. Therefore, the use of cylinder is becoming more popular,particularly in the research laboratories.

Cylinders are cast and tested in the same position, whereas cubes are cast in onedirection and tested from the other direction. In actual structures in the field, the casting andloading is similar to that of the cylinder and not like the cube. As such, cylinder simulates thecondition of the actual structural member in the field in respect of direction of load.

The points in favour of the cube specimen are that the shape of the cube resembles theshape of the structural members often met with on the ground. The cube does not requirecapping, whereas cylinder requires capping. The capping material used in case cylinder mayinfluence to some extent the strength of the cylinder.

The Flexural Strength of ConcreteConcrete as we know is relatively strong in compression and weak in tension. In

reinforced concrete members, little dependence is placed on the tensile strength of concretesince steel reinforcing bars are provided to resist all tensile forces. However, tensile stresses arelikely to develop in concrete due to drying shrinkage, rusting of steel reinforcement,temperature gradients and many other reasons. Therefore, the knowledge of tensile strengthof concrete is of importance.

A concrete road slab is called upon to resist tensile stresses from two principal sources–wheel loads and volume change in the concrete. Wheel loads may cause high tensile stresses


due to bending, when there is an inadequate subgrade support. Volume changes, resultingfrom changes in temperature and moisture, may produce tensile stresses, due to warping anddue to the movement of the slab along the subgrade.

Stresses due to volume changes alone may be high. The longitudinal tensile stress in thebottom of the pavement, caused by restraint and temperature warping, frequently amountsto as much as 2.5 MPa at certain periods of the year and the corresponding stress in thetransverse direction is approximately 0.9 MPa. These stresses are additive to those producedby wheel loads on unsupported portions of the slab.

Determination of Tensile StrengthDirect measurement of tensile strength of concrete is difficult. Neither specimens nor

testing apparatus have been designed which assure uniform distribution of the “pull” applied


to the concrete. While a number ofinvestigations involving the directmeasurement of tensile strength have beenmade, beam tests are found to bedependable to measure flexural strengthproperty of concrete.

The value of the modulus of rupture(extreme fibre stress in bending) depends onthe dimension of the beam and manner ofloading. The systems of loading used infinding out the flexural tension are centralpoint loading and third point loading. In thecentral point loading, maximum fibre stresswill come below the point of loading wherethe bending moment is maximum. In case ofsymmetrical two point loading, the criticalcrack may appear at any section, not strongenough to resist the stress within the middle

third, where the bending moment is maximum. It can be expected that the two point loadingwill yield a lower value of the modulus of rupture than the centre point loading. Figure 10.4shows the modulus of rupture of beams of different sizes subjected to centre point and thirdpoint loading. I.S. 516-1959, specifies two point loading. The details of the specimen andprocedure are described in the succeeding paragraphs.

The standard size of the specimens are 15 x 15 x 70 cm. Alternatively, if the largestnominal size of the aggregate does not exceed 20 mm, specimens 10 x 10 x 50 cm may beused.

The mould should be of metal, preferably steel or cast iron and the metal should be ofsufficient thickness to prevent spreading or warping. The mould should be constructed withthe longer dimension horizontal and in such a manner as to facilitate the removal of themoulded specimens without damage.

The tamping bar should be a steel bar weighing 2 kg, 40 cm long and should have aramming face 25 mm square.

Flexural testing of concrete beam mould

Principles of flexural testing


The testing machine may be of any reliable type of sufficient capacity for the tests andcapable of applying the load at the rate specified. The permissible errors should not be greaterthat ± 0.5 per cent of the applied load where a high degree of accuracy is required and notgreater than ± 1.5 per cent of the applied load for commercial type of use. The bed of thetesting machine should be provided with two steel rollers, 38 mm in diameter, on which thespecimen is to be supported, and these rollers should be so mounted that the distance fromcentre to centre is 60 mm for 15 cm specimen or 40 cm for 10.0 cm specimens. The load isapplied through two similar rollers mounted at the third points of the supporting span, thatis, spaced at 20 or 13.3 cm centre to centre. The load is divided equally between the twoloading rollers, and all rollers are mounted in such a manner that the load is applied axiallyand without subjecting specimen to any torsional stresses or restrains. The loading set up isshown in Fig. 10.5.

ProcedureTest specimens are stored in water at a temperature of 24° to 30°C for 48 hours before

testing. They are tested immediately on removal from the water whilst they are still in a wetcondition. The dimensions of each specimen should be noted before testing. No preparationof the surfaces is required.

Placing the Specimen in the Testing MachineThe bearing surfaces of the supporting and loading rollers are wiped clean, and any

loose sand or other material removed from the surfaces of the specimen where they are tomake contact with the rollers. The specimen is then placed in the machine in such a mannerthat the load is applied to the uppermost surface as cast in the mould, along two lines spaced20.0 or 13.3 cm apart. The axis of the specimen is carefully aligned with the axis of the loadingdevice. No packing is used between the bearing surfaces of the specimen and the rollers. Theload is applied without shock and increasing continuously at a rate such that the extreme fibrestress increases at approximately 0.7 kg/sq cm/min that is, at a rate of loading of 400 kg/minfor the 15.0 cm specimens and at a rate of 180 kg/min for the 10.0 cm specimens. The loadis increased until the specimen fails, and the maximum load applied to the specimen duringthe test is recorded. The appearance of the fractured faces of concrete and any unusualfeatures in the type of failure is noted.

The flexural strength of the specimen is expressed as the modulus of rupture fb which if‘a’ equals the distance between the line of fracture and the nearer support, measured on thecentre line of the tensile side of the specimen, in cm, is calculated to the nearest 0.05 MPaas follows:

fb = P l

b d×× 2

When ‘a’ is greater than 20.0 cm for 15.0 cm specimen or greater than 13.3 cm for a10.0 cm specimen, or

fb = 3

2p a

b d××

when ‘a’ is less than 20.0 cm but greater than 17.0 cm for 15.0 specimen, or less than 13.3cm but greater than 11.0 cm for a 10.0 cm specimen where

b = measured width in cm of the specimen,


d = measured depth in cm of the specimen at the point of failure,

l = length in cm of the span on which the specimen was supported, and

p = maximum load in kg applied to the specimen.

If ‘a’ is less than 17.0 cm for a 15.0 cm specimen, or less than 11.0 cm for a 10.0 cmspecimen, the results of the test be discarded.

As mentioned earlier, it is difficult to measure the tensile strength of concrete directly. Oflate some methods have beenused with the help of epoxybonded end pieces to facilitatedirect pulling. Attempts have alsobeen made to find out directtensile strength of concrete bymaking briquette of figure 8shape for direct pulling but thismethod was presenting somedifficulty with grip andintroduction of secondary stresseswhile being pulled.

Whatever may be themethods adopted for finding outthe ultimate direct tensilestrength, it is almost impossible toapply truly axial load. There isalways some eccentricity present. The stresses are changed due to eccentricity of loading.These may introduce major error on the stresses developed regardless of specimen size andshape.

The third problem is the stresses induced due to the grips. There is a tendency for thespecimen to break near the ends. This problem is always overcome by reducing the section

Splitting tensile test


of the central portion of the test specimen. The method in which steel plates are glued withthe epoxies to the ends of test specimen, eliminates stresses due to griping, but offers nosolution for the eccentricity problem.

All direct tension test methods require expensive universal testing machine. This explainswhy these tests are not used on a routine basis and are not yet standardised.

Indirect Tension Test MethodsCylinder Splitting Tension Test: This is also sometimes referred as, “Brazilian Test”. This test

was developed in Brazil in 1943. At about the same time this was also independentlydeveloped in Japan.

The test is carried out by placing a cylindrical specimen horizontally between the loadingsurfaces of a compression testing machine and the load is applied until failure of the cylinder,along the vertical diameter. Figure 10.6 shows the test specimen and the stress pattern in thecylinder respectively.

When the load is applied along the generatrix, an element on the vertical diameter ofthe cylinder is subjected to a vertical compressive stress of

2PLD!

Dr D r

21

( )−−

and a horizontal stress of

2PLD!

where, P is the compressive load on the cylinder

L is the length of cylinder

D is its diameter

and r and (D – r) are the distances of the elements from the two loads respectively.

The loading condition produces a high compressive stress immediately below the twogenerators to which the load is applied. But the larger portion corresponding to depth issubjected to a uniform tensile stress acting horizontally. It is estimated that the compressivestress is acting for about 1/6 depth and the remaining 5/6 depth is subjected to tension.

In order to reduce the magnitude of the high compression stresses near the points ofapplication of the load, narrow packing strips of suitable material such as plywood are placedbetween the specimen and loading platens of the testing machine. The packing strips shouldbe soft enough to allow distribution of load over a reasonable area, yet narrow and thinenough to prevent large contact area. Normally, a plywood strip of 25 mm wide, 3 mm thickand 30 cm long is used.

The main advantage of this method is that the same type of specimen and the sametesting machine as are used for the compression test can be employed for this test. That is whythis test is gaining popularity. The splitting test is simple to perform and gives more uniformresults than other tension tests. Strength determined in the splitting test is believed to be closerto the true tensile strength of concrete, than the modulus of rupture. Splitting strength givesabout 5 to 12% higher value than the direct tensile strength.


Ring Tension Test 10.17

Another test for finding out the tensile strength of concrete is known as “Ring TensionTest”. Briefly in this method, a hydrostatic pressure is applied radially against the insideperiphery of 15 cm diameter, 4 mm thick and 4 mm high concrete ring specimen. Theresulting tensile stress developed in the specimen are determined from the equations of thestress analysis of thick walled cylinders, as given below:

ft = P r

r rrr

i i

i

−

−−

2

02 2

02

21

where, ft = Tensile strength

pi = Applied hydrostatic pressure

ri = Internal radius

r0 = External radius

r = Radius at point of failure

Advantages of Ring Tension TestThe nature of the load application in this test is such that no clamping and misalignment

stresses are introduced in the test specimen, a condition difficult to avoid in direct tests.

The entire volume of the ring is subjected to tensile stresses with the uniformly distributedmaximum stress occurring along the entire periphery of the ring. This is never achieved in theflexural tests and even in the cylinder splitting test a compressive load acting on a diametralplane creates a uniform tensile stress over that plane only.

The magnitude of the radial compressive stress is quite small when compared with thetangential stress. This is a definite advantage over the splitting tension test in which theminimum compressive stresses occurring at the centre line of the splitting plane is about threetimes the corresponding tensile stress.

Limitations of Ring Tension TestThe drawbacks of this test are that here also, the derivation of equations used for the

stress analysis is based upon Hook’s Law of linear stress-strain proportionality. The ring tensilestrengths obtained appear to be somewhat higher than the true tensile strength of concrete,the magnitude of the exact difference has yet to be firmly established.

Double Punch Test 10.16

Yet another test to find out the indirect tensile strength of concrete is known as “DoublePunch Test”. In this test, a concrete cylinder is placed vertically between the loading plates ofthe compression test machine and compressed by the steel punches located concentrically onthe top and bottom surfaces of the cylinder.

An ideal failure mechanism will consist of many simple tension cracks in radial directionand two cone shaped rupture surfaces directly under the loads. Two cone shapes movetowards each other as a rigid body and displace the surrounding material horizontallysideways. The formulae for calculating the tensile strength has been calculated on the basisof limit analysis. The relation is :

ft = Q

bH a( . )120 2−


where, a = radius of punch

b = radius of cylinder

H = height of the cylinder

Q = load at failure

Factors influencing the strength resultsIt has already been pointed out that shape and size of specimen affects the strength

results. If the strength of the 15 cm size cube is taken as standard, then the strength of 10 cmcube should be reduced by 10%. Strength of the cylinder of size 15 cm diameter and 30 cmlong is taken as 0.8 of the strength of 15 cm cube. Where cubes larger than 15 cm areadopted, generally no modification to the strength is necessary unless otherwise specified.

The planeness of the end condition of specimen and capping material used for thecylinder affects the strength. The employment of lubricating material at the bearing surfaceof the sample affect the strength of concrete.

The effect of height to diameter ratio has already been discussed.

The rate of application of load has a considerable affect on the apparent strength ofconcrete; the lower the rate of application of load, the lower will be the recorded strength.The reason for this is probably the effect of creep. If the load is applied slowly, or in theapplication of load, if there is some time-lag, the specimen will undergo certain amount ofcreep which will increase the strain. The enhanced strain due to creep will be responsible forfailure of the sample, at a lower value of stress applied. The Figure 10.7 shows the influenceof rate of application of load on the compressive strength of concrete.

The state of moisture content of the specimen influences the observed strength to a greatextent. If two cubes made from identical concrete, one is wet and another is dry, if tested atthe same age, the dry cube gives higher strength than the wet cube. It is quite probable thatthe dry cube may have undergone drying shrinkage which will have ultimately caused someamount of drying shrinkage cracks and bond failures. From this simple reason it must give animpression that dry cube must give a lower value, but on the contrary the result is the otherway. The probable explanation is that due to wetting, some sort of dilation of cement gel willtake place by the adsorbed water. The forces of cohesion of the solid particles are then


decreased. Perhaps the decrease of strength on account of reduction of cohesion owing tothe adsorbed water may be more than that of the loss of strength due to rupture of gel bondson account of drying shrinkage.

To have a standard condition for test specimens, it is usual to test a specimen immediatelyon removal from the curing water tank. This condition has the advantage of being betterreproducible than a dry condition which includes greatly varying degrees of dryness.

It was earlier pointed out that contrary to expectations, the wet concrete exhibits highermodulus of elasticity. Strength and modulus of elasticity do not go hand in hand in the caseof wet concrete.

Test CoresThe test specimen, cube or cylinder is made from the representative sample of concrete

used for a particular member, the strength of which we are interested. As the member cannot be in fact tested, we test the parallel concrete by making cubes or cylinders. It is to beunderstood that the strength of the cube specimen cannot be same as that of the memberbecause of the differences with respect to the degree of compaction, curing standard,uniformity of concrete, evaporation, loss of mixing water etc. At best the result of cube orcylinder can give only a rough estimate of the real strength of the member.

To arrive at a better picture of the strength of the actual member, attempts are made tocut cores from the parent concrete and test the cores for strength. Perhaps this will give abetter picture about the strength of actual concrete in the member.


Core can be drilled at the suspected part of the structure or to detect segregation orhoney combing or to check the bond at construction joint or to verify the thickness ofpavement.

The disadvantages are that while cutting the core, the structural integrity of the concreteacross the full cross-section may be affected to some extent and secondly that the diameterto height ratio may be other than that of the standard cylinder. Capping of both ends will berequired which will again introduce some differences in the strength. Existence ofreinforcement will also present difficulty in cutting a clean core.

The cores cut to determine the strength of concrete of the actual structure may alsoindicate segregation and honey combing of concrete. In some cases, the beam specimens arealso sawn from the road and airfield slabs for finding flexural strength. In practice, it is seenthat the strength of the core is found to be less than that of the strength of standard cylinders.Apart from other reasons, it is mainly because site curing is invariably inferior to curing understandard moist condition.

Strength of CoresThe reduction in strength of cores appear to be greater in stronger concrete. The

reduction in the strength can be as high as 15 per cent for 40 MPa concrete. Generally, areduction of 5 to 7 per cent is considered reasonable. It has been reported by manyinvestigators that in situ concrete gains very little strength after 28 days. Tests on high strengthconcrete show that, although the strength of cores increase with age, the core strength, evenup to the age of 1 year, remains lower than the strength of standard 28 day cylinders. Fig.10.8 and Table. 10. 2 illustrates the above statements.

Table 10.2. Development of strength of Cores with Age

Age Strength Core strength as adays MPa proportion of strength of

Standard Cores 28-day standardcylinders cylinder

7 66.0 57.9 0.72

28 80.4 58.5 0.73

56 86.0 61.2 0.76

180 97.9 70.6 0.88

365 101.3 75.4 0.94

Non-Destructive Testing MethodsNon-destructive methods have been in use for about four decades. In this period, the

development has taken place to such an extent that it is now considered as a powerfulmethod for evaluating existing concrete structures with regard to their strength and durabilityapart form assessment and control of quality of hardened concrete. In certain cases, theinvestigation of crack depth, microcracks, and progressive deterioration are also studied by thismethod.

Though non-destructive testing methods are relatively simple to perform, the analysis andinterpretation of test results are not so easy. Therefore, special knowledge is required to


analyses the hardenedproperties of concrete. Inthe non-destructivemethods of testing, thespecimen are not loadedto failure and as such thestrength inferred orestimated cannot beexpected to yield absolutevalues of strength. Thesemethods, therefore,attempt to measure someother properties ofconcrete from which anestimate of its strength,durabil ity and elasticparameters are obtained. Some such properties of concrete are hardness, resistance topenetration of projectiles, rebound number, resonant frequency and ability to allow ultrasonicpulse velocity to propagate through it. The electrical properties of concrete, its ability to absorb,scatter and transmit X-rays and Gamma-rays, its response to nuclear activation and its acousticemission allow us to estimate its moisture content, density, thickness and its cement content.Based upon the above, various non-destructive methods of testing concrete have beendeveloped:

1. Surface hardness tests: These are of indentation type, include the Williams testingpistol and impact hammers, and are used only for estimation of concrete strength.

2. Rebound test: The rebound hammer test measures the elastic rebound of concreteand is primarily used for estimation of concrete strength and for comparativeinvestigations.

3. Penetration and Pull out techniques: These include the use of the Simbi hammer, Spitpins, the Windsor probe, and the pullout test. These measure the penetration andpullout resistance of concrete and are used for strength estimations, but they can alsobe used for comparative studies.

4. Dynamic or vibration tests: These include resonant frequency and mechanical sonicand ultrasonic pulse velocity methods. These are used to evaluate durability anduniformity of concrete and to estimate its strength and elastic properties.

5. Combined methods: The combined methods involving ultrasonic pulse velocity andrebound hammer have been used to estimate strength of concrete.

6. Radioactive and nuclear methods: These include the X-ray and Gamma-raypenetration tests for measurement of density and thickness of concrete. Also, theneutron scattering and neutron activation methods are used for moisture and cementcontent determination.

7. Magnetic and electrical methods: The magnetic methods are primarily concerned withdetermining cover of reinforcement in concrete, whereas the electrical methods,including microwave absorption techniques, have been used to measure moisturecontent and thickness of concrete.

A view of rotary core cutting drill with necessary arrangement, readyto extract 50 mm dia core.


8. Acoustic emission techniques: These have been used to study the initiation andgrowth of cracks in concrete.

9. Surfaces Hardness Methods: The fact that concrete hardens with increase in age, themeasure of hardness of surface may indicate the strength of concrete. Variousmethods and equipments are devised to measure hardness of concrete surface.William testing pistol, Frank spring hammer, and Einbeck pendulum hammer are someof the devices for measuring surface hardness.

Schmidt’s Rebound HammerSchmidt’s rebound hammer developed in 1948 is one of the commonly adopted

equipments for measuring the surface hardness. The sectional view of the hammer is shownin Figure 10.9.

It consist of a spring control hammer that slides on a plunger within a tubular housing.When the plunger is pressed against the surface of the concrete, the mass rebound from theplunger. It retracts against the force of the spring. The hammer impacts against the concreteand the spring control mass rebounds, taking the rider with it along the guide scale. Bypushing a button, the rider can be held in position to allow the reading to be taken. Thedistance travelled by the mass, is called the rebound number. It is indicated by the ridermoving along a graduated scale.

Each hammer varies considerably in performance and needs calibration for use onconcrete made with the aggregates from specific source. The test can be conductedhorizontally, vertically—upwards or onwards or at any intermediate angle. At each angle therebound number will be different for the same concrete and will require separate calibrationor correction chart. Fig.10.11 shows the typical relationship between compressive strengthand rebound number with hammer horizontal and vertical on a dry or wet surface ofconcrete.

Limitation:Limitation:Limitation:Limitation:Limitation: Although, rebound hammer provides a quick inexpensive means of checkinguniformity of concrete, it has serious limitations and these must be recognised. The results areaffected by:

(a) Smoothness of surface under test.

(b) Size, shape and rigidity of the specimen.

(c ) Age of specimen.

(d ) Surface and internal moisture condition of the concrete.

(e ) Type of coarse aggregate.


(f ) Type of cement.

(g ) Type of mould.

(h) Carbonation of concrete surface.

Rebound Number and Strength of ConcreteInvestigations have shown that there is a general correlation between compressive

strength of concrete and rebound number; however, there is a wide degree of disagreement

Concrete test hammer,normal

Concrete test hammer,digital

Testing anvil


among various research workers regardingthe accuracy of estimation of strength fromrebound readings. The variation ofstrength of a properly calibrated hammermay lie between ±15% and ±20%.

The relationship between flexuralstrength and rebound number is found tobe similar to those obtained forcompressive strength, except that thescatter of the results is greater. Fig. 10.11shows the relationship betweencompressive strength of concrete cylindersand rebound numbers.10.6

During 1965 and 1967 an international survey on the use of the Schmidt reboundhammer was carried out by RILEM. Majority of those who spoke were against the use ofSchmidt rebound hammer in acceptance testing. The consensus was that, “the Schmidtrebound hammer is useful to very useful in checking uniformity of concrete and comparingone concrete against another but it can only be used as a rough indication of concretestrength in absolute terms”.

Penetration TechniquesThe measurement of hardness by probing techniques was first reported during 1954.

Two techniques were used. In one case, a hammer known as, “Simbi” was used to perforate

A view of Rebound Hammer connected with digitalrecording monitor and Test Anvil for calibration.


concrete and the depth of borehole was correlated to compressive strength of concrete cubes.In the other technique, the probing of concrete was achieved by blasting with spit pins andthe depth of penetration of the pins was correlated with compressive strength of concrete.


The accuracy of this test was found to be ±25%. However, it is further seen that, “Simbi” andspit pins were more effected by the arrangement of coarse aggregate, than the tests usingrebound hammers.

During 1964 and 1966, a technique known as the “Windsor Probe” was advanced fortesting concrete in the laboratory and in situ. The windsor probe is a hardness tester of thesurface of the concrete. It is an equipment consisting of a powder activated gun, hardenedalloy probes, loaded cartridges, and depth gauge for measuring penetration of probes. Theprobe is driven into the concrete by firing of a precision powder charge cartridges. Theexposed length is measured by calibrated depth gauge and this is correlated to the strengthof concrete cylinders. Fig. 10.12 shows the relationship between exposed probe length and28 day-compressive strength.10.7

The windsor probe test cannot be really considered as a non-destructive testing, as itmakes a hole and damages the structure. It can only be considered non-destructive to theextent that concrete can be tested in situ and structural members. In case of big structures likepavements or retaining walls etc., the structure need not be discarded.

The Windsor probe test is basically a hardness tester and, like other hardness testers,should not be expected to yield accurate absolute values of strength of concrete in a structure.However, like the “Schmidt rebound hammer”, the probe test provides a good method fordetermining the relative strength of concrete in the same structure or relative strength indifferent structures without extensive calibration with specific concrete.


Pullout testA pullout test measures the force required to pull out from the concrete a specially shaped

rod whose enlarged end has been cast into that concrete. The stronger the concrete, the moreis the force required to pullout. The ideal way to use pullout test in the field would be toincorporate assemblies in the structure. These standard specimens could then be pulled outat any point of time. The force required denotes the strength of concrete. Another way to usepullout test in the field would be to cast one or two large blocks of concrete incorporatingpullout assemblies. Pullout test could then be performed to assess the strength of concrete.Figure 10.14 shows the relationship between compressive strength and pullout strength. 10.8

Dynamic or Vibration MethodsThis is the important non-destructive method used in testing concrete strength and other

properties. The fundamental principle on which the dynamic or vibration methods are basedis velocity of sound through a material. A mathematical relationship could be establishedbetween the velocity of sound through specimen and its resonant frequency and therelationships of these two to the modulus of elasticity of the material. The relationships whichare derived for solid mediums considered to be homogeneous, isotropic and perfectly elastic,but they may be applied to heterogeneous materials like concrete.

The velocity of sound in a solid can be measured by determining the resonant frequencyof specimen or by recording the time of travel of short pulses of vibration passing through thesamples. In non-destructive testing of concrete, either resonance method or pulse velocitytechniques could be adopted. Figure 10.15 shows dynamic methods of testing concrete.


Resonant Frequency MethodThis method is based upon the

determination of the fundamentalresonant frequency of vibration of aspecimen.

The resonance is indicated by thepoint of maximum amplitude for thevarious driving frequencies generated.The equipment used for this is usuallyknown as ‘Sonometer”. Resonantfrequency methods are mostly used inthe laboratory. Figure 10.16 showsschematic diagram of a typicalapparatus.

Usefulness of Resonant Frequency MethodThese tests can normally be carried out only on small sized specimens in a laboratory

rather than on structural members in the field. The possibility of vibrating structural membersat resonance is neither practical nor desirable. The size of specimens in these tests is usuallylimited to 150 x 300 mm cylinders or 75 x 75 x 300 mm prisms.

The equations for the calculation of dynamic modulus involve “shape factor” corrections.This necessarily limits the shape of the specimens to cylindrical or prismatical types. Anydeviation from the standard shapes can render the application of shape factor correctionsrather complex.

Notwithstanding the above limitations, the resonance tests do provide an excellent meansfor studying the deterioration of concrete specimens subjected to repeated cycles of freezingand thawing and to deterioration due to acidic and alkali attack. The use of resonance testsin the determination of damage by fire has also been reported.

Ultrasonic Pulse Velocity Test Equipment


The resonant frequency test results are often used to calculate dynamic Young’s modulusof elasticity of concrete but the values obtained are somewhat higher than those obtainedwith standard static tests carried out at lower rates of loading. The use of dynamic Young’smodulus in design calculations is not recommended.

Various investigators have published correlations between the strength of concrete andits dynamic modulus of elasticity. (Figure 10.17). The indiscriminate use of such correlationsto predict compressive and/or flexural strength of concrete is strongly discouraged unlesssimilar relationships have been established in the laboratory for the particular concrete underinvestigation.

Pulse Velocity MethodThis can be sub-divided into two parts:

(a) Mechanical sonic pulse velocity method, which involves measurement of the time oftravel of longitudinal or compressional waves generated by a single impact hammerblow or repeated blows.

(b) Ultrasonic Pulse Velocity Method, which involves measurement of the time of travelof electronically generated mechanical pulses through the concrete.

Out of these two, the ultrasonic pulse method has gained considerable popularity all overthe world. When mechanical impulses are applied to a solid mass, three different kinds ofwaves are generated. These are generally known as longitudinal waves, shear waves andsurface waves. These three waves travel at different speeds. The longitudinal or compressionalwaves travel about twice as fast as the other two types. The shear or transverse waves are notso fast, the surface waves are the slowest.

The pulses can be generated either by hammer blows or by the use of an electroacoustictransducer. Electroacoustic transducers are preferred as they provide better control on the typeand frequency of pulses generated. The instrument used is called “Soniscope”.

Ultrasonic pulse velocity method consists of measuring the time of travel of an ultrasonicpulse, passing through the concrete to be tested. The pulse generator circuit consists ofelectronic circuit for generating pulses and a transducer for transforming these electronicpulses into mechanical energy having vibration frequencies in the range of 15 to 50 kHz. Thetime of travel between initial onset and the reception of the pulse is measured electronically.The path length between transducer divided by the time of travel gives the average velocityof wave propagation.


Recently, battery operated fully portable digitised units have become available in U.K. Onesuch unit is called “PUNDIT” (Protable Ultrasonic Non-destructive Digital-Indicating Tester.). Itonly weights 3 kgs.10.10

Techniques of Measuring Pulse Velocity through ConcreteThere are three ways of measuring pulse velocity through concrete. They are:

(a) Direct transmission.

(b) Indirect transmission.

(c ) Surface transmission.

Figure 10.18 shows the methods of measuring.

Factors Affecting the Measurement of Pulse VelocityThe measurement of pulse velocity is affected by a number of factors regardless of the

properties of concrete.

1. Smoothness of contact surface under test: Smoothness of contact surface under test: Smoothness of contact surface under test: Smoothness of contact surface under test: Smoothness of contact surface under test: It is important to maintain good acousticalcontact between the surface of concrete and the face of each transducer. Generally this doesnot pose any problem because in normal testing sufficient cast surfaces are available for goodcontact. However, when it is necessary to hold the transducer against an unmoulded surface,for example, the top surface of a test cylinder, it is desirable to smoothen the surface by theuse of a carborundum stone and the transducers should be held tightly against the concrete


surface. In addition, the use of a coupling medium such as a thin film of oil, soap, jelly, orkaolin glycerol paste should be used.

2. Influence of path length on pulse velocity: Influence of path length on pulse velocity: Influence of path length on pulse velocity: Influence of path length on pulse velocity: Influence of path length on pulse velocity: As concrete is inherently heterogeneous,it is essential that path length be sufficiently long so as to avoid any errors introduced due toits heterogeneity. In field work, this does not pose any difficulty because pulse velocitymeasurements are generally carried out on thick structural concrete members, where the pathlengths may be anywhere from 300 mm in the case of columns to 23 m in mass gravity dams.However, in the laboratory where generally small specimens are used, the path length canaffect the pulse velocity readings.

3. T T T T Temperaturemperaturemperaturemperaturemperature of concre of concre of concre of concre of concrete: ete: ete: ete: ete: It has been reported that variations of the ambienttemperature between 5° C and 30° C do not significantly affect the pulse velocitymeasurements in concrete. At temperatures between 30° C and 60°C, there is up to 5%reduction in pulse velocity. This is probably due to the initiation of microcracking in concrete.At below freezing temperature, the free water freezes within concrete thus resulting in anincrease in pulse velocity. At 4° C, an increase of up to 7.5% in the pulse velocity throughwater-saturated concrete has been reported. The detailed corrections to pulse velocitymeasurements due to change in temperature are given in Table 10.3.

Table 10.3. Corrections to Pulse Velocity Measurements due tochanges in Temperature

Temperature Correction per cent

Air-dried Water-saturated °C concrete concrete

+60 +5.0 +4.0

+50 +3.5 +2.8

+40 +2.0 +1.7

+20 0.0 0.0

0 – 0.5 – 1.0

– 4 & below – 1.5 – 7.5

4. Moistur4. Moistur4. Moistur4. Moistur4. Moisture condition of concre condition of concre condition of concre condition of concre condition of concrete:ete:ete:ete:ete: In general, pulse velocity through concrete increaseswith increased moisture content of concrete. This influence is more marked for low-strengthconcrete than high strength concrete. It is considered that the pulse velocity of saturatedconcrete may be about 2% higher than that of similar dry concrete.

5. Pr5. Pr5. Pr5. Pr5. Presence of resence of resence of resence of resence of reinforeinforeinforeinforeinforcing steel: cing steel: cing steel: cing steel: cing steel: The presence of reinforcing steel in concrete considerablyaffects the pulse velocity measurements because pulse velocity in steel is 1.2 to 1.9 times thevelocity in plain concrete, thus pulse velocity measurements taken near the steel reinforcingbars may be high and may not represent the true velocity in concrete.

Generally, it is desirable to choose pulse paths that avoid the influence of reinforcing steel.However, when it is not possible to do so, the measured values have to be corrected by takinginto account the proximity of the pulse path to the reinforcing steel, the quantity andorientation of the steel with respect to the propagation path, and the pulse velocity in theconcrete.


When the axis of reinforcing bars is perpendicular to the direction of propagation andthe quantity of reinforcement is small, the influence of the reinforcement on the pulse velocityis generally small. The correction factors are of the order of 1– 4% depending upon the qualityof the surrounding concrete the higher the quality of concrete, the smaller the correctingfactor.

When the axis of the reinforcing bars is parallel to the direction of propagation of thepulse, the influence of reinforcement cannot be avoided easily. The pulse velocitymeasurements can be corrected but correction factors are of approximate nature only, and incase of two-way reinforcement it is almost impossible to make any reliable corrections.

Accuracy of MeasurAccuracy of MeasurAccuracy of MeasurAccuracy of MeasurAccuracy of Measurement.ement.ement.ement.ement. It is generally agreed that the ultrasonic concrete testermeasures the transit time through small specimens with an accuracy of 0.1 microseconds; forthe specimens of the same length, the accuracy of measurement for the soniscope and thePUNDIT is of the order of 0.5 microseconds. Thus, the former instrument is ideally suited forcontrolled laboratory studies, whereas the latter are best suited for field investigations wherethe path lengths are longer.

ApplicationsThe pulse velocity methods have been used to evaluate concrete structures and attempts

have been made to correlate the pulse velocity with strength and other properties of concrete.The various applications of the pulse velocity methods are described below:

Establishing uniformity of concrEstablishing uniformity of concrEstablishing uniformity of concrEstablishing uniformity of concrEstablishing uniformity of concrete: ete: ete: ete: ete: For establishing the uniformity of concrete, theultrasonic concrete tester is an ideal tool for laboratory specimens, whereas the soniscope andPUNDIT provide an excellent means for both laboratory and field studies.

Establishing acceptance criteria:Establishing acceptance criteria:Establishing acceptance criteria:Establishing acceptance criteria:Establishing acceptance criteria: Generally, high pulse velocity reading in concrete areindicative of concrete of good quality. Table 10.4 gives the pulse velocity ratings, as given byLeslie and Cheesman.

Table 10.4. Suggested Pulse Velocity for Concrete10.11

Pulse velocity General conditions(m/s)

4575 ... ... ... ... Excellent3660—4575 ... ... ... ... Good3050—3660 ... ... ... ... Questionable2135—3050 ... ... ... ... Poor 2135 ... ... ... ... Very poor

Table 10.5. Velocity Criterion for Concrete Quality Grading(As per IS : 13311-Part I)

SL. No. Pulse velocity by cross-probing, km/sec. Concrete quality greading

1. Above 4.5 Excellent2. 3.5 to 4.5 Good

3. 3.0 to 3.5 Medium

4. Below 3.0 Doubtful


Table 10.6. Velocity Criterion for Concrete Quality Grading by SurfaceProbing (As per NCBM)

SL. No. Pulse velocity by surface probing, km/sec. Concrete quality grading

1. Above 3.5 Excellent2. 3.0 to 3.5 Good

3. 2.5 to 3.0 Medium

4. Below 2.5 Poor

Determination of pulse modulus of elasticity:Determination of pulse modulus of elasticity:Determination of pulse modulus of elasticity:Determination of pulse modulus of elasticity:Determination of pulse modulus of elasticity: Theoretically, the values of the pulsemodulus of elasticity calculated from the readings obtained with the soniscope or theultrasonic concrete tester should be the same as those obtained with resonant frequencytechniques. However, this has not been found to be so.

For this reason and also because the modulus of elasticity depends upon density andPoisson’s ratio, most researchers have attempted to use pulse velocity itself as a criterion of thequality of concrete without attempting to calculate moduli therefrom. If it is desired to computemodulus of elasticity from the pulse velocity, the formulae given under Chapter 8 should beused.

Estimation of strengthEstimation of strengthEstimation of strengthEstimation of strengthEstimation of strengthof concrof concrof concrof concrof concre te :e te :e te :e te :e te : Variousresearchers have attemptedto correlate compressiveand flexural strength ofconcrete with pulse velocity.Fig 10.19 show relationshipbetween pulse velocity andcompressive strength forvarious aggregate/cementratio. 10.12

Determinat ion ofDeterminat ion ofDeterminat ion ofDeterminat ion ofDeterminat ion ofsetting characteristics ofsetting characteristics ofsetting characteristics ofsetting characteristics ofsetting characteristics ofconcre te :concre te :concre te :concre te :concre te : Thedetermination of the rate ofsetting of concrete bymeans of the soniscope hasbeen widely used.

Studies on durability ofStudies on durability ofStudies on durability ofStudies on durability ofStudies on durability ofconcre te :concre te :concre te :concre te :concre te : Durabil ity ofconcrete under freeze-thawaction and the aggressiveenvironments such assulphate attack and acidicwaters, have been studiedby various investigatorsusing the pulse velocitytechnique to assessdamage.


Pulse velocity techniquesPulse velocity techniquesPulse velocity techniquesPulse velocity techniquesPulse velocity techniques have been successfully used for the measurement and detectionof cracks. The basic principle of crack detection is that, if the crack is of appreciable width andconsiderable depth, perpendicular to the test path, no signal will be received at the receivingtransducer. If the depth of the crack is small, compared to the distance between thetransducers, the pulse will pass around the end of the crack and the signal is received at thetransducer. However, in doing so, it would have travelled a distance longer than the straightline path upon which pulse velocity computations are made. The difference in the pulsevelocity is then used to estimate the path length and hence crack depth. Figure 10.20 showsthe principle involved in measurements of crack depth.

Measurement of deterioration of concrete due to fire exposure: Measurement of deterioration of concrete due to fire exposure: Measurement of deterioration of concrete due to fire exposure: Measurement of deterioration of concrete due to fire exposure: Measurement of deterioration of concrete due to fire exposure: Pulse velocity techniqueshave been used to estimate the deterioration due to fire. In one of the experiments, prismsof the size 88 x 102 x 406 mm have been exposed to fire for 1 hour at temperatures rangingfrom 100° to 1000°C. After the exposure, the specimens were removed from the furnace andallowed to cool to room temperature. Pulse velocity was then measured using the ultrasonicconcrete tester, following this, the prisms were tested in flexure. The per cent loss in pulse


velocity followed very closely the per cent loss in flexural strength of test prisms after fireexposure.

Determination of the time of removal of form work can be sometimes assessed bymeasuring the pulse velocity. Figure 10.21 shows the relationship between age and pulsevelocity for different water/cement ratio.

Pulse velocity techniques have also been used for the measurement of thickness ofconcrete pavements.

Relationship between Pulse Velocity and Static Young’s Modulus ofElasticity

An empirical relationship between pulse velocity and Static Young’s Modulus of elasticityis shown in Figure 10.22. Using this relationship, it may be possible to estimate the value ofYoung’s modulus elasticity at those points in a structure, where pulse velocity measurementsare taken. It may be remembered that pulse modulus which is called “Dynamic Modulus” hasbeen discussed earlier. Dynamic modulus is invariably higher than static modulus.

Combined MethodsOne of the most important objectives of non-destructive methods of testing of concrete

is to estimate the compressive strength of concrete in structure. Use of any one method maynot give reliable results. Using more than one method at the same time has been found togive reliable results regarding the strength of a structure. Most popular combination was foundto be the ultrasonic pulse velocity method in conjunction with the hardness measurementtechniques, and Rebound Hammer method. Fig.10.23 shows the relationship between PulseVelocity and Rebound number for various strength of concrete.

Radioactive MethodsThe use of X-rays and gamma-rays as non-destructive method for testing properties of

concrete is relatively new. X-ray and gamma-rays both components of the high energy region


on the electromagnetic spectrum penerate concrete but undergo attenuation in the process.The degree of attenuation depends on the kind of matter traversed, its thickness, and thewavelength of the radiation. The intensity of the incident gamma-rays and the emerginggamma-rays after passing through the specimens are measured. These two values are madeuse of for calculating the density of structural concrete members.

Gamma-rays transmission method has been used to measure the thickness of concreteslabs of known density. Gamma radiation source of known intensity is made to pass andpenetrate through the concrete. The intensity at the other face is measured. From thisthickness of the concrete is calculated.

Nuclear MethodsUse of nuclear methods for non-destructive measurement of some properties of concrete

is of recent origin. Two principal techniqueshave been reported, namely neutronscattering methods for determining themoisture content of concrete and neutronactivation analysis for the determination ofcement content. These methods are notsuitable for finding out the strength ofconcrete.

Magnetic MethodsBattery operated magnetic devices that

can measure the depth of reinforcementcover in concrete and detect the position of

A view of Profometer with attached Scanner tolocate the reinforcements and to determinespacing, diameter and concrete cover.


reinforcement bars are now available. The apparatus is known as cover meter. This can beused for measuring the cover given in the lightly reinforced sections.

Electrical MethodsRecently some electrical

methods have been employed fordetermining the moisture content ofhardened concrete, tracing ofmoisture permeation throughconcrete and determining thethickness of concrete pavements.

The accurate determination ofthe moisture content of hardenedconcrete is required in connectionwith creep, shrinkage and thermalconductivity studies. The fact thatdielectric properties of hardenedconcrete change with changes inmoisture content is made use of inthis method.

Electrical resistivity methods have been used to find out the thickness of concretepavements. The method is based on the principle that the material offers resistance to thepassage of an electric current. A concrete pavement has a resistivity characteristic that isdifferent from that of the underlying subgrade layers. A change in the slope of the resistivityverses depth curve is used to estimate the depth of concrete pavement.

Tests on Composition of Hardened ConcreteSometimes the dispute regarding the quality of work may arise between contractor and

department and this dispute may be submitted to arbitration. In such cases, it will be necessarythat the composition of the hardened cement concrete may be required to be ascertained bychemical analysis.

Determination of CementContent

The method of finding out thecement content is based on the factthat the silicates in Portland cementare much more readily decomposedand made soluble in dilutehydrochloric acid than the solubilityof silica contained in the aggregates.Similarly, the l ime contained incement is much more soluble thanlime content in aggregates, with theexception of lime stone aggregates.

The procedure is to crush arepresentative sample of the concrete

A view of Resistivity meter well connected with Wennerfour probe, ready to measure the resistivity of concrete.

A view of Corrosion Analyser for half-cell potentialmeasurement with the help of copper/copper sulphatereference electrode.


and dehydrate it at a temperature of 550°C for 3 hours. A small portion of the sample is takenand treated with 1:3 hydrochloric acid. The quantity of silica is determined by standardchemical methods.

The filtrate from the silica determination contains soluble calcium oxide from theaggregate and the cement, and further calculations depend on whether or not the aggregateis largely siliceous. If the original aggregate is available its solubility should be tested.

From the contents of soluble silica and calcium oxide the cement content in the originalvolume of the sample can be calculated, using A.S.T.M. Standard method C 85-66. Theseresults are reliable and can be used to check the cement content of different parts of astructure, e.g., when it is desired to establish whether or not segregation has taken place. Theaccuracy of the test is, however, lowest for mixes with low cement contents, and, it is oftenin this type of mix that the exact value of cement content is required. Furthermore, the testdepends on the knowledge of the chemical composition of aggregate, which may not beavailable for testing. When large amounts of both soluble silica and calcium oxide are liberatedfrom the aggregate, the method is not reliable.

More complex techniques are prescribed by B.S. 1881 : Part 6 : 1971 but it should benoted that chemical tests are rather, expensive and are used only in resolving disputes, andnot as a means of control of the quality of concrete.

Determination of the Original Water/Cement RatioA method of estimating the water/cement ratio that existed at the time of placing of a

concrete mix, now hardened, has been developed by Brown. In essence, this involvesdetermining the volume of the capillary pores and the weight of cement and combined water.

A sample of concrete is oven-dried at 105°C and the air is removed from the pores undervacuum. The pores are then refilled with carbon tetrachloride, whose weight is measured, andhence the weight of the water which originally occupied the pores can be calculated. Sincethe voids formed through air entrainment are discontinuous they remain filled with air whenthe vacuum is applied and no water is absorbed in them. The result of the test is thusunaffected by entrained air.

The sample is now broken up, the carbon tetrachloride having been allowed toevaporate, and the aggregate is separated out and weighed. The loss on ignition and the CO2

content of the remaining fine material are determined, and from these two quantities theweight of combined water can be calculated.

The sum of the combined water and the pore water gives the original mixing water. Thequantity of anhydrous cement can also be determined either in conjunction with this test orby the method described earlier and hence the water/cement ratio of the mix can becalculated within about 0.02 of the true value. The technique has been developed into astandard method of B.S. 1881 : Part 6 : 1971.

Physical MethodPolivka 10.15 has successfully used a “Point-count” method on a sawn and varnished

surface of a dried concrete specimen to determine its cement content, total aggregatecontent, and fine/coarse aggregate ratio. The basis of the method is the fact that the relativevolumes of the constituents of a heterogeneous solid are directly proportional to their relativeareas in a plane section, and also to the intercepts of these areas along a random line.Furthermore, the frequency with which a constituent occurs at a given number of equally


spaced points along a random line is a direct measure of the relative volume of that constituentin the solid. Thus point-count by means of a stereo-microscope can rapidly give the volumetricproportions of a hardened concrete specimen.

The aggregate and the voids (containing air or evaporable water) can be identified, theremainder being assumed to be hydrated cement. In order to convert the quantity of the latterto the volume of unhydrated cement, we have to know the specific gravity of dry cement andthe non-evaporable water content of hydrated cement.

The test determines the cement content of the concrete within 10 per cent but theoriginal water content or voids ratio cannot be estimated since no distinction is made in thetest between air and water voids.

Accelerated Curing TestConcrete is usually tested at the age of 7 days and 28 days. Concrete mixes are designed

usually for 28 days strength and sometimes for 7 days strength. One will come to knowwhether the concrete has attained specified strength or not, at the end of 28 days or 7 days.At the work site, normally, concrete is placed daily. In some situations, concrete is poured daily,over the previoulsy laid concrete, as in the case of columns, retaining walls etc. The progressof laying concrete cannot be held up for 28 days or even 7 days until the strength isascertained. When the concrete strength is known, after 28 days and if it happens to beinferior concrete, not satisfying the required strength, the engineer will be put to anembarrassing situation. Sometimes, it may so happen, that the upper lift may satisfy thestrength requirement but the lower one may fail to satisfy the strength requirement. Some ofthe suggestions given to overcome such situations are devaluation of work, dismantling of allthe works, redesigning and reducing the load of the structure above the level of such weakconcrete, grouting of concrete etc. It can be said that none of the above suggestions aresatisfactory.

Perhaps, finding out the strength of concrete in 8 hours time and correlating it to 28 daysstrength is the best method to overcome such situations. If one comes to know, in about 8hours time, the 28 days strength, if the concrete does not satisfy, steps could be taken withoutany legal complication by replacing the faulty concrete. The problem is only to find out the28 days strength, reliably in about 8 hours.

Many research workers worked in this directions to depict 28 days strength within a shortperiod. Out of all the procedures, accelerated curing test developed by Prof. King is consideredto be more accurate and easy for adoption.

In this test, standard concrete cubes are made. These cube moulds are covered both ontop and bottom by cover plates and sealed with the grease at the contact surface, to preventescaping of moisture from concrete. Cubes are placed in an airtight oven within 30 minutesof the addition of water and then switched on. The temperature is brought on to 93°C inabout an hour. The cubes are kept at this temperature for a period of 5 hours. Then the cubesare stripped off and allowed to cool within half an hour. The total time spent is 7 hours. Atthe end of 7 hours the cube is crushed. Seven days or 28 days strength of the concrete isdeduced from the standard curve established giving the relationship between King’saccelerated curing test results and 7 and 28 days strength. The relationship is shown in Figure7.4. 10.18


R E F E R E N C E S10.1 Neville A.M., The Failure of Concrete Compression Test Specimens, Civil Engineering July, 1957.

10.2 Murdock J.U. and Kesler C.L., Effect on Length to Diameter Ratio of Specimen on the ApparentCompressive Strength of Concrete ASTM Bulletin, April 1957.

10.3 Wright PJF, The Effect of the Method of Test on the Flexural Strength of Concrete, Magazine ofConcrete Research, Vol 4 No. II.

10.4 Mehenry D and Shideler JJ, Review of Data on effect of Speed in Mechanical Testing of Concrete,A.S.T.M. special Technical Publication No. 185, 1956.

10.5 R.D. Gaynor, One look at concrete compressive Strength, NRMCA Publication No. 147 of Nov. 1974.

10.6 Willetts C.H. Investigation of the Schmidt concrete Test Hammer, Miscellaneous Paper No. 6-627, U.S.Army Engineer Waterways Experiment Station, June 1958.

10.7 Arni H.T. Impact and Penetration Tests of Portland Cement Concrete, Highway Research Record No.378 of 1972.

10.8 U.Bellander, Strength in Concrete Structures, CBI Report, Sweedish Cement and Concrete ResearchInstitute 1978.

10.9 Sharma M.R. and Gupta B.L., Sonic Modulus is Related Strength and static Modulus of high strengthConcrete, The Indian Concrete Journal, April 1960.

10.10 Pundit Manual, CNS Instrument Limited, 61-63 Holmes Road London. N.W. 5.

10.11 Leslie J.R. and Cheesman W.J., An Ultrasonic Method of Studying Deterioration and Cracking inConcrete Structures ACI Journal, Sept. 1949.

10.12 Jones R, Non Destructive Testing of Concrete, Cambridge University Press London 1962.

10.13 Elvery R.H. and Forrester J.A., Non-Destructive Testing of Concrete, Progress in Construction, Scienceand Technology, 1971.

10.14 N.J. Cario, Non Destructive Testing of Concrete: History and Challenges. Proceedings of MohanMalhotra Symposium, ACI SP 144–1994.

10.15 Polivka M. and Kelly J.W, A Physical Method for Determining the Composition of Hardened ConcreteA.S.T.M. Sp. Technical Publication 205 1958.

10.16 Chen W.F., Double Punch Test for Tensile Strength of Concrete, Proc ACI No. 12 Vol-67, Dec. 1970.

10.17 Malhotra V.M., Problems Associated with Determining the Tensile Strength of Concrete, Transactionsof the Engineering Institute of Canada, Vol 10 July 1967.

10.18 King JUH, Further notes on Accelerated Test for Concrete, Chartered Civil Engineer, May 1957.

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